1. Field of the Invention
The present invention relates to a vehicle steering system for use in automobiles and other motor vehicles, and in particular to such a steering system which is suitable for steering the rear wheels of a four wheel steering vehicle and provided with a sensor for indicating its steering output.
2. Description of the Relevant Art
In the conventional steering systems for motor vehicles, in particular those for four wheel steering vehicles, for the purpose of controlling the steering angle, it is customary to use a steering angle sensor typically consisting of a differential transformer to detect a longitudinal movement of a steering rod which is guided by a housing assembly for the longitudinal movement determined by an output from a control unit. This is disclosed, for instance, in copending U.S. patent application Ser. No. 07/563,413 filed Aug. 6, 1990.
In such a steering angle sensor, the moveable rod of the differential transformer extending perpendicularly to the steering rod is brought into contact with a slope defined on a side face of an indicator member which moves integral with the steering rod, and the axial movement of the steering rod is converted into the axial movement of the moveable rod of the differential transformer by the slope. Obviously, the longitudinal displacement of the steering rod is directly associated with the steering angle of the steered wheels.
However, for satisfactory operation of such a steering angle sensor, it is necessary to ensure a high level of precision in the distance between the guide for the steering rod and the guide for the indicator member, and these two guides must be accurately parallel to each other. Further, the distance and the angular relationship between the steering rod and the indicator member must be accurate. These considerations result in the increase in the cost for fabrication, and the reduction in the efficiency of the process of fabrication.
In such a vehicle steering system, a steering rod is coaxially passed through an electric motor, and the steering rod is axially actuated by the electric motor via a ball and nut mechanism. The steering rod is coupled to a tie rod for directly actuating a wheel carrier, via a ball joint or the like. The ball and nut mechanism is an efficient means to convert a rotational movement into an axial movement, but it is inevitable that some rotational force is applied to the steering rod, and it is quite detrimental to the durability of the steering rod, in particular the durability of the couplings connected thereto.
Therefore, it is necessary for the steering rod to be provided with means for preventing the rotational movement thereof around its central axial line. To this end, it is possible to employ a radial arm extending radially from the steering rod, and guiding the free end of the arm strictly in the axial direction or in parallel with the axial line of the steering rod as disclosed in the above mentioned copending application. However, for satisfactory operation of such means for preventing the rotation of the steering rod, the free end of the radial arm is required to be guided in a highly smooth manner yet without any substantial play, and this requires a high level of precision which means an increase in the manufacturing cost.